The flow diagram in FIG. 1 schematically shows a light source L including, for example, one or more LEDs L1, L2, L3, . . . (connected in series, in the illustrated example), organized in one or more cells, to which a respective switch can be coupled, for example an electronic switch S1, S2. The switch is connected to the respective LED cell in such a way that, when the switch is open, the current Iout coming from a power supply source 10 flows through the LED or the LEDs of the cell, which are therefore energized, whereas when the switch is closed (i.e. conductive), current flows through the switch itself and not through the LED or the LEDs, which are therefore de-energized.
By controlling the opening and closing of the switch or switches S1, S2, for example by varying the duty cycle of a rectangular waveform driving the opening and closing of the switch or switches, and as a consequence by varying the duration of the time intervals during which the individual LED cells are either energized or short-circuited by the respective switch, and therefore de-energized, it is possible to correspondingly vary the output light intensity. The foregoing takes place according to known criteria, so as not to require a more detailed explanation herein.
In implementing dimming solutions of the kind schematically shown in FIG. 1, it is desirable to have a current source 10 with a very rapid dynamic response, so as to be able to maintain the output current Iout flowing on the LEDs as steady as possible. This is true even in the case of a rapid variation in time of the number of LEDs that are energized at every given instant.
In order to meet this requirement, the generator 10 should behave as an ideal current generator, able to maintain the same output current value Iout wholly irrespective of the variations in the load instantly constituted by LEDs L1, L2, L3, . . . and by the switches S1, S2, . . . coupled thereto.
In the state of the art there are known direct current driving solutions for light sources such as LEDs, wherein the power source is comprised of a Switching Mode Power Supply, SPMS.
Specifically, solutions are known which are based on a feedback mechanism, wherein a signal representative of the current flowing through the load is used as a driving variable of a control loop. The design of the controller used can be for example the one known as PI (proportional/integral control), or else the design known as PID (proportional/integral/derivative).
In various known solutions, the bandwidth of the current feedback loop is maintained definitely below (approx. one tenth or less) the switching frequency of the power supply source 10 (typically in the range of 20-200 kHz). The ability to perform the dimming action is attained only through a duty-cycle modulation, used for driving the LEDs.